Frequency



United States PatentO 2,733,341 TRAVELING WAVE MAGNETRON CIRCUITS Philip H. Peters, Jr., Schenectady, N. Y., assignor to the United States of America as represented by the Secretary of the Army 1 Application September 16, 1953, Serial No. 380,625 13 Claims. (Cl. 250-36) The invention relates in general to improvements in magnetron circuits, and more particularly to improvements in voltage-tunable traveling wave magnetron circuits.

As is well known in the art, in a conventional traveling wave magnetron a synchronous, radio-frequency electric field is induced in a tuned circuit-by the rotating, spokeshaped, space charge cloud of electrons Whose configuration and rotation are governed by the anode voltage, magnetic field, cathode emission and magnetron geometry. The oscillating frequency of the magnetron is determined chiefly by the resonant frequency of the tuned circuit coupled thereto. For a more detailed explanation of traveling wave magnetrons, reference is made to Radar Systems and Components by Bell Laboratories Staff, pages 6685 (1949).

With such a traveling wave magnetron, if the potential between the cathode and the anode is increased, the angular velocity of the electron space charge cloud also momentarily increases. Also, there is an increase in the energy input to the tuned circuit, which results in an increased alternating high frequency field. While the increase in the anode voltage tends to increase the angular velocity of the electron space charge cloud, the increased R. F. electric field tends to decrease said velocity to keep the space charge cloud in step with the center frequency of the tuned circuit. However, the compensation is not complete and the frequency of oscillation fails to return to its original value, but increases slightly with the anode voltage. This phenomenon is known conventionally in the art as frequency pushing.

A method of operating a traveling wave magnetron so that the retarding effect of the fringing field is minimized and the oscillating frequency is mainly dependent on the angular velocity of the space charge cloud and in turn upon the anode voltage and magnetic field is discussed in a copending application of Philip H. Peters, Jr., and Donald Aldrich Wilbur, Serial No. 169,712, filed June 22, 195 0. In that application, a resonant circuit is shown coupled across the sections of a conventional, split anode, traveling wave magnetron, and a load impedance is connected across the resonant circuit. With such a magnetron, loading the resonant circuit and/ or restricting the emission from the magnetron cathode results in a large increase in the magnetron oscillating frequency when the anode-to-cathode voltage is increased; that is to say, an increase in the pushing of the magnetron is achieved. Eventually a point of loading and restricted emission is reached where the pushing effect of the anode voltage completely dominates the forces tending to hold the electron space charge in synchronism with the center-frequency of the resonant circuit. In other words when the rate at which the R. F. field grows with increasing D. C. anode voltage becomes insufiicient to compensate for the increase in cloud velocity, a transition to voltage tunable operation takes place. The frequency of operationof themagnetron is then no longer dependent upon the resonant circuit but is determined by the anode-to-cathode potential and the magnetic field strength. This transition occurs under a condition of heavy loading. The value of the resistive component of the resonant circuit determines the R.F. power level generated. Either the anode-to-cathode voltage or the strength of the magnetic field, or both, may be varied to control the frequency of operation of the magnetron. The foregoing is discussed in detail in the above mentioned copending application.

Several limitations were encountered in operating such voltage tunable, traveling wave magnetrons over wide tuning ranges. The power output was found to vary with frequency because of the relatively rapid decrease in the impedance of the resonant circuit on either side of resonance. Lowering the cathode emission in order to minimize the synchronizing effect of the retarding R. F. field reduced the anode current and resulted in a lowered power output. On the other hand, an attempt to overcome the synchronizing forceswithout lowering the cathode emission required a vane loading impedance so low as to permit the production of only a small amount of R. F. power.

The present invention represents an improvement upon the magnetron of the copending application above referred to, in that it provides means for obtaining a more uniform power output over a broader band of frequencies. These advantages are achieved by providing a plurality of stagger-tuned, resonant circuits, the capacitive element of each circuit including only a portion of the total vane-tocathode capacitance of the magnetron. Also, by tuning the resonant circuits to different, widely separated frequencies, several frequencies may be generated simultaneously.

It is therefore an important object of this invention to provide a voltage tunable traveling wave magnetron having an increased tuning range and a more uniform power output level over that tuning range than was heretofore attainable with a two-terminal, voltage-tunable magnetron using a single resonant circuit.

Another object of the present invention is to provide a traveling wave magnetron capable of simultaneously generating a plurality of signals at different frequencies.

A still further object of this invention is to accomplish the aforementioned objects by means of a simple and inexpensive magnetron structure.

The features of my invention which I belive to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawing in which: 7

Fig. 1 represents a schematic circuit diagram embodying the principles of my invention;

Figs. 2 and 3 are graphs of impedance and power output versus frequency in megacycles of the embodiment in Fig. 1;

Fig. 4 is a schematic circuit of another embodiment of my invention;

Fig. 5 is a graph of impedance and power output versus frequency of the embodiment in Fig. 4.

In Fig. 1 there is shown a split anode magnetron 1 having two vanes 3 symmetrically disposed about a cathode 7 Although only two vanes are shown, it should be understood that each vane 3 may comprise a plurality of fingers, the fingers of the two vanes 3 being interdigitated. Cathode 7 is connected to a terminal 2 going to B-minus, and is also connected through a radio frequency bypass condenser 31 to ground. A pair of resonant circuits 5A and 5B are respectively connected between the vanes or vane sets 3 and ground. Between these resonant circuits 5A and 5B is a grounded shield 9, which is preferably adjustable, for controlling the mutual coupling between the resonant circuits, although any other means for varying the coupling may be used. A magnetostatic B-field 25 is positioned axially with cathode 7. Although the frequency of the output signals of the magnetron may be tuned by adjusting the parameters of the resonant circuits A and 5B, the invention is particularly applicable to the case where the magnetron is tunable by variation of its anodecathocle potential and/or the magnetic field.

Fig. 2 depicts the output signals generated by circuits 5A and 58 at their center frequencies, line A corresponding to circuit 5A and line B corresponding to circuit 53. The shield 9 is adjusted so that the mutual coupling between resonant circuits 5A and 5B is low enough for the magnetron shown in Fig. l to generate two frequencies simultaneously. As shown in Fig. 2, a total separation of about 50% of the average of the two frequencies of the two resonant circuits 5 may be achieved. Thus, by the use of my invention, two separate signals may be generated at useful power levels, one of which could be used as a radar carrier and the other of which could be used as an interrogation signal. The anode voltage is set to produce electron cloud synchronization near the average of the two frequencies.

As shown in Fig. 3 by the line labeled A, B, when the coupling between the tuned circuits 5A and 5B is increased, the two signals will eventually synchronize to form a single coherent signal. The frequency separation among these signals may be varied to some extent by changing the anode voltage, magnetic field, or coupling between circuit elements. Comparing Fig. 3 with Fig. 2, it will be apparent that greater power output is developed by the magnetron when the coupling between the tuned circuits is sutficiently close to cause them to oscillate at a single frequency.

By properly fixing the bandwidth and coupling of the resonant circuits it is also possible to vary the frequency separation between the two signals with the anode voltage alone. When this is done, one signal will move in frequency and the other will remain essentially constant.

Referring now to Fig. 4, there is shown a more complete embodiment of the present invention in which a four vane, traveling wave, voltage-tunable magnetron 1 is shown as having an envelope 29. Four vanes 3 are syrnmetrically disposed about a cyclindrical cathode 7. As in Fig. 1, each vane may comprise a pluarlity of fingers which are connected together and made interdigital with the fingers of the other vanes. A heater 4 is disposed within cathode '7, and is connected through a pair of lines 23 and a rheostat 21 across a source of heater potential 1%. The cathode 7 is connected to the movable arm 33 of a potentiometer 11, which is placed across a source of potential 13. The positive end of the source 13 is connected to ground at a terminal 15. An R. F. bypass capacitor 31 is connected between cathode 7 and ground. To the vanes 3 are respectively connected circuits 5A, 5B, 5C and 5D. These resonant circuits 5 have low Qs, preferably of or less. Each resonant circuit includes an inductance, a capacitance, and a resistance, and the respective inductances are variably link-coupled to coils 17A, 17B, 17C and 17D. The latter are connected in series and between output terminals 27, one of which terminals is connected to ground at a terminal 16. Three adjustable grounded shields 9 are disposed between the four resonant circuits 5A, 5B, 5C and 5D, and are used to control the mutual coupling between the resonant circuits. A magnetostatic B-field is positioned axially with cathode 7.

The magnetron 1 shown in Fig. 4 may be made voltage tunable by adjusting the coupling between the coils 17 and the resonant circuits 5 so as to load circuits 5 until the transition previously mentioned is reached. Upon reaching this transition, the magnetron may be voltage tuned by moving arm 33 of potentiometer 11 so as to vary the anode-to-cathode potential between the vanes 3 and the cathode '7. The frequency can also be varied by varying the magnitude of the B-field 25. The power output of the magnetron may be varied with only a minor variation in frequency with rheostat 21 which controls the amount of voltage applied by source 19 to the heater 4 of cathode 7. However, the cathode emission should not be allowed to exceed the value which will permit voltage tunable operation. Preferably, this condition is established by both loading the tuned circuits 5 and lowering the emission of cathode '7.

As was explained in connection with Fig. 1, variable shields 9, in the embodiment of Fig. 4, effectively vary the bandpass characteristics of circuits 5A, 5B, 5C and 51). Further, varying the coupling between coils 17A, 17B, 17C and 17D with respect to circuits 5A, 5B, 5C and 51) will respectively vary the power output level of each tuned circuit.

Referring now to Fig. 5, a graph of impedance of power output versus frequency for each of the circuits 5A, 5B, 5C and SD of Fig. 5, is shown. Curves A, B, C, an D p sen he output of circuits 5 5B, 5 an 5D, respectively. The couplings between coils 17A-17D and the circuits 5A5D are adjustable so that the power output and impedances of each circuit may be made equal. Further, shields 9 are adjustable so that the coupling between circuits 5 may be varied until the resonance curves of the circuits overlap and form a stagger-tuned network. When the power levels and mutual coupling of circuits 5A5D are so adjusted, a resultant, substantially constant output power level and impedance will be achieved over a wide frequency range, as indicated by dashed line 35.

It is desired to emphasize the fact that although resonant circuits may be used when it is desired to staggertune a voltage-tuned magnetron, or produce two or more separate frequencies, or produce a single frequency, at high frequencies these circuits may be replaced by elements having distributed constants. Carbon resistors have been found to serve well as self-resonant circuits.

While there has been described what is at present considered a preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. In combination, a traveling wave magnetron having a cathode and at least two anode vanes, and at least two resonant means coupled together and respectively connected between each of said vanes and said cathode and means interposed between said resonant means for varying the mutual coupling therebetween to vary the band pass characteristic of said resonant means.

2. The combination set forth in claim 1, wherein said resonant means are tuned to different frequencies.

3. The combination set forth in claim 1, wherein said resonant means are tuned to the same frequency.

4. In combination, a traveling wave magnetron having a cathode and a plurality of anode vanes, an equal plurality of resonant circuit means coupled together and respectively connected between each of said vancs and said cathode and means interposed between said circuit means for varying the mutual coupling therebetween to vary the band pass characteristic of said' resonant circuit means.

5. The combination set forth in claim 4, further comprising a common output circuit coupled to said resonant circuit means.

6. In combination, a traveling wave magnetron having a cathode coupled toa point of reference potential and a plurality of anode vanes symmetrically disposed about said cathode, an equal plurality of tuned circuit means coupled together and respectively connected between said vanesand said point of reference potential, adjustable shield means interposed between said circuit means for varying the mutual coupling between said circuit means to vary the band pass characteristics of said circuit means,

and coupling means for variably coupling each of said circuit means to a common output element.

7. The combination set forth in claim 6, wherein all of said circuit means are tuned to the same frequency.

8. The combination set forth in claim 6, wherein said circuit means are respectively tuned to different frequencies.

9. The combination set forth in claim 6, wherein said band pass circuit means are center-tuned to different frequencies separated from one another by an amount equal to fifty percent of the average frequency.

10. The combination set forth in claim 6, wherein said resonant circuit means are center-tuned to sucessively higher frequencies and whose band pass characteristics overlap.

11. In combination, a voltage tunable, traveling wave magnetron having a cathode and at least two anode vanes, and at least two resonant means coupled together and respectively connected between said vanes and said cathode, said anode vanes being energized by a variable unidirectional potential means connected between said vanes References Cited in the file of this patent UNITED STATES PATENTS 2,099,300 Fritz Nov. 16, 1937 2,169,725 Fritz Aug. 15, 1939 2,243,202 Fritz May 27, 1941 2,270,160 Berger Jan. 13, 1942 

